This invention relates generally to automated monolithic circuit design techniques and, more particularly, to the design of microwave or similar circuits in which physical dimensions and circuit topology have a significant effect on the electrical characteristics and performance parameters of the circuits.
For some types of circuitry, such as relatively low-frequency digital circuitry, the physical topology of a circuit has little, if any, effect on circuit performance and characteristics. At higher frequencies, and especially in microwave circuits, the shape of circuit components, such as transmission lines, and their proximity to each other have a very significant effect on performance. Parasitic impedances between adjacent components, radiation losses from sharply curved transmission lines, and unexpected effects of scaling components in size, all contribute to the difficulty of microwave circuit design.
The process of designing low-frequency digital circuit design has been automated to some degree for a number of years. There, the circuit layout problem is generally a matter of fitting a desired set of circuit components into as small an area as possible and arranging that interconnecting conductive traces are laid out in a manner that minimizes the lengths of the conductive paths. There is usually little concern about conductor spacing, except that there is usually a minimum spacing geometry dictated by the resolution limits of the fabrication process employed. Another aspect of low-frequency design is that a skilled designer can typically predict circuit performance with considerable accuracy from a knowledge of the circuit layout.
By way of contrast, the performance of monolithic microwave circuitry is much more difficult to predict from the circuit layout. Currently, the design of even a simple microwave monolithic circuit requires repeated iterations by an expert circuit designer. Even with the availability of computer software to aid the designer in some areas, the process still depends heavily on the expert designer's ability to make appropriate iterative changes in the design, to translate the output from one design aid program for input to another, and to perform other manual steps in reaching an optimum design, frequently on an intuitive basis.
Typically, the first step in the circuit design process is synthesis, to determine the initial topology and element values of matching components that will be used to perform a desired circuit function. Circuit synthesis computer programs are commercially available, to synthesize various microwave circuits, such as filters and amplifiers. The next step is optimization, in which the values provided in the initial synthesis process are optimized for better performance. Computer programs are also available to perform circuit optimization for various microwave circuits. The final step before fabrication is layout of the circuit. This step relies heavily on the expert designer to come up with a circuit topology that conforms with the synthesized and optimized circuitry, but does not contribute adversely to circuit performance. Multiple iterations of the layout step are almost always required and, although computer simulations of performance are helpful, the final design may not be achieved without several iterations of fabrication and testing of the resulting circuitry. A particularly important aspect of the layout step is circuit compaction. As in all monolithic circuit design, an important goal is to minimize the overall circuit area. In low-frequency design, compaction of the circuit into as small an area as possible is a tedious but relatively simple problem. In microwave circuit design, the problem is far from trivial. For example, a transmission line in a microwave circuit must be of a specified length. The space occupied by the transmission line can be reduced by introducing multiple bends into the line, but this results in at least two related difficulties: radiative losses from bends that are too sharp, and interference, physically or electrically, betweem adjacent components.
Because of the complexity and repetitive nature of the microwave circuit design task, even simple circuits may take a highly skilled designer weeks to complete. Therefore, it will be appreciated that there is a need for an alternative approach that minimizes the involvement of expert designers, and produces completed circuit designs efficiently and rapidly. The present invention satisfies this need.